Data processing systems have generally been developed to provide system configurations which range from compact, single-board microcomputers to more complex, high performance minicomputers. Such systems use microcode architecture in which macroinstructions are suitably decoded so as to provide access to a microinstruction or to a sequence of more than one microinstruction obtained from a suitable data store thereof.
Generally, in such systems, for example, a macroinstruction is appropriately supplied from a macroinstruction register to suitable decoding logic so as to provide a starting address for access in the microinstruction data store (sometimes referred to as the microcode store) of an initial microinstruction of a sequence thereof. The accessed microinstruction includes control information for performing the instruction designated and sequence information for determining the microaddress of the next microinstruction of the sequence. Each sequential microinstruction contains the same kind of information until the last microinstruction of the sequence has been accessed at which point the microinstruction routine having been completed, the system is ready to decode the next macroinstruction.
Such systems normally require a relatively large microcode data store utilizing microinstruction words which are relatively wide (i.e., they contain a relatively large number of bits) so as to contain the required control and sequencing information. While the use of relatively wide microinstruction words provides higher speed operation (i.e., a large number of bits are simultaneously available in parallel to provide the control and sequencing operations) such systems tend to be more costly not only because the number of storage bits in the microcode data store becomes relatively high but the data paths for handling a wide microinstruction word become more complex and the system requires more expensive components and data path configurations.
In order to reduce the data storage space required for the microinstructions and to avoid handling a large number of "wide" instruction words, certain microcode systems have utilized "two-level" microcode store techniques as opposed to one level microcode stores as discussed above. Such two-level configurations arise from the recognition that control information in the microinstruction words are often common to a large number of the microinstructions. Therefore, in order to avoid the repetitive storage of the same relatively large number of data bits required to store all of the control and sequencing information for each microinstruction separately, control information, which is common to many microinstructions, is stored in one ROM store separately from sequencing information which is stored in a different ROM store. At the "first level" of operation the sequencing process is performed at the sequence microcode store ROM to produce sequential addresses to access control information in the control microcode store ROM which at the "second level" of operation provides the control information required to sequentially perform the particular microinstruction involved, which latter information may be common to many microinstructions.
Such two-level approach tends to reduce the microcode storage space required in comparison with one-level microcode systems. Such two-level technique, however, may tend to provide only a limited microcode operational capability since the limited control store capacity may permit the system to be designed for a specified set of microinstructions with no real flexibility for providing for expansion of the basic microinstruction set in order to increase the operational capability of the overall system.